The present invention relates to an apparatus and a method for moulding plastic preforms into plastic containers and in particular into plastic bottles. Such apparatus and methods have been known from the prior art for a long time. In this respect, heated plastic preforms are usually moulded in blowing stations by applying pressurised air onto plastic bottles. To this end, the plastic preforms are introduced into a blow mould, which is then closed, and in this closed condition the expansion of the plastic preforms is carried out. One problem that frequently occurs in connection with this moulding process is the interface between the two blow mould parts, for example blow mould halves, which during production can lead to undesired seams on the finished container.
It is therefore known in the prior art to use so-called pressure pads in such moulding stations, which pressure pads press, during the expansion of the plastic preforms, the two blow mould parts or blow mould halves onto each other. In this approach which is known from the applicant's internal prior art, the same pressure that during the blow moulding process is passed into the container and is therefore prevalent therein, is also applied to the pressure pad. Therefore, the pressure profile in the pressure pads corresponds here to the blowing pressure profile. As a result, always only the pressure that is required at any given moment for keeping the blow mould shut so that there is no mould gap is applied to this pressure pad. The mould carrier will never be stressed beyond that.
It is further known from the applicant's internal prior art to provide a separate pressure pad control which ensures that pressure is already applied onto the pressure pad prior to the blowing process, which pressure is even higher than the final blowing pressure. In the course of this, the pressure pad pressure is kept constant during the entire blowing process. This embodiment is especially relevant in the case of large-volume containers, where the projected area would build up such a great force that the final blowing pressure is no longer sufficient to keep the moulds shut or against each other. The disadvantage of this approach consists in higher energy consumption and in the fact that the load on the mould carrier is very high.
In a further approach known from the applicant's internal prior art, the pressure pad is actuated using a lower pressure compared to the final blowing pressure. This is also referred to as pressure pad reduction. Pressure is already applied to the pressure pad prior to the blowing process. The pressure pad pressure continues to be kept constant during the entire blowing process. This embodiment is particularly useful for small-volume containers, and in this case the stress on the mould carrier can be significantly reduced.
In a further method known from the prior art, in which the pressure profile of the pressure pad corresponds exactly with the blowing pressure profile, the pressure pad will not move until after the shaping process of the container has already started. However, this may lead to a seam to be formed in the container, because the mould gap has not yet been closed.
The present invention is therefore based on the object of avoiding on the one hand, as far as possible, a bottle seam to be formed and moreover also of achieving a gentle movement of the mould halves and a lower stress on the mould carrier.